Uninterruptible power supplies or systems, commonly referred to as UPS, are used to provide backup power to critical loads such as computer systems, industrial microprocessor controlled systems, and the like, where a loss of line power can result in the interruption of programs and the loss of valuable data or a system malfunction. The UPS may also provide a line power conditioning function to ensure that transient spikes, low voltage conditions, or distorted power waveforms on the AC power system do not disturb the operation of the computer or other system which is supplied with power through the UPS. Typically, the UPS includes a battery which is interfaced through an inverter to the AC output line. Various designs for backup or uninterruptible power systems have been developed. Generally, it is desirable that the power supplied to the critical load from the backup power system should be without a substantial break or discontinuity of power flow despite breaks in the main AC power system power. Systems which can provide power without observable breaks or discontinuities in the waveform of the voltage supplied to the critical load despite loss or disruptions of the AC input power are often complex and relatively expensive.
Lower cost backup power systems have been introduced which are intended for use with individual desk top computer systems rather than larger (and more expensive) computers or local area networks. These less expensive types of backup power systems commonly utilize a mechanical relay as the means by which power is transferred from the AC power system to the battery powered inverter. As a consequence, the power transfer is typically slow, with an often significant break in the power supplied to the critical load. Such systems are sometime referred to as "standby power supplies" or SPS in distinction from true uninterruptible power supplies in which there is no significant break in the output voltage waveform. A simplified block diagram for a typical backup power system of this type is illustrated in FIG. 1. The backup power system receives power from the main AC power system 20 at its input terminals 21 and supplies power to a critical load 22 from its output terminals 23 and 24. A relay coil 25 is connected across the two AC power lines 26 and 27 of the backup power system, and this relay coil is coupled to a first set of relay contacts 28 and a second relay contact 29. The relay contacts 28 switch from a first position, illustrated in FIG. 1, in which the relay 28 supplies power from the input terminals 21 on the power line 26 to the output terminal 23. In the second position of the switch 28, power is supplied to the relay 28 from an inverter 30 which receives DC input power from a battery 31 through the relay switch 29 when it is closed. When AC input power is available across the input terminals 21, the relay coil 25 is energized, which holds the relay contacts 28 and 29 in their position shown in FIG. 1. When the AC input power from the power system 20 fails, the relay coil 25 is de-energized, causing the relay contact 29 to close to provide DC power from the battery 31 to the inverter 30, and causing the relay contact 28 to switch to connect the output of the inverter 30, which is now turned on to supply AC output power, through the contact 28 to the output terminals 23 and 24 to supply power to the load 22. When the AC power system 20 comes back on line and again supplies AC power to the terminals 21, the relay coil 25 is again energized, switching the relay contacts 28 and 29 so that power is again supplied to the load from the main AC power system. Because of the time required to fully de-energize the relay coil 25 and the relatively slow operating time of the mechanical switches 28 and 29, a substantial break in the output power supplied to the load is inevitably encountered both when switching the supply of power from the power system to the inverter and when switching back again from the inverter to the power system. Because of the way the relay coil is activated, such systems generally do not provide switching of power during events other than complete power failure, such as overvoltages from the power system 20 or low voltages (brownouts).
A loss of power of several milliseconds can be tolerated by some loads which have the capability of riding through the power loss, for example, conventional incandescent lamps or motors. Some types of electronic equipment have a power supply with a relatively large input power supply filter which has sufficient stored energy to supply the consuming load during the time required to transfer power from the power system to the inverter and vice versa.
There are also many types of critical loads for which a break in the input voltage waveform of as short as two milliseconds can cause the load to fail. For example, high intensity discharge lights such as those used in many hospital operating rooms may extinguish upon loss of power for a relatively short period of time, in the range of a few milliseconds, and may require a significant period of time (e.g., several minutes) before they can be restarted. Modern computers and other types of electrical equipment often include power factor correcting power supply circuits which are designed to improve the power factor which is presented to the AC power line. Such power factor corrected power supplies typically produce an input current waveform which tracks the shape and phase of the AC input voltage., to provide a corrected power factor that approaches unity. Power factor correcting power supplies which utilize an active high frequency correction system may not tolerate disruptions in AC input power very well. In certain cases, a break in input power flow of the length typical of standby power systems as illustrated in FIG. 1 can cause instabilities and oscillations in the power factor corrected power supply, and a consequent failure of a computer receiving power from the power supply.
A variation of the backup power supply design of FIG. 1 which also has been utilized is shown in FIG. 2. In this system, a line voltage detector 35 is connected by lines 36 and 37 to the AC power lines 26 and 27. The line voltage detector 35 is connected to the relay coil 25 and also to the inverter 30. The detector 35 monitors the AC input power across the lines 26 and 27. When a power fault is detected, in accordance with the specifications of the line voltage detector, the detector de-energizes the relay coil 25 to switch the position of the relay 28 from its first position, as shown in FIG. 2, in which power is supplied from the AC power system to the load, to its second position in which it connects the load 22 to the inverter 30. The line voltage detector 35 is also connected to the inverter 30 to turn it on at the same time that it is de-energizing the relay coil 25. Because the switch 28 can only connect either the input line 26 to the output terminal 23 (to supply AC power from the AC power system to the load 22), or connect the inverter 30 to the output terminal 23 (to connect the inverter to the load), the inverter 30 and the AC power system 20 are never connected together in parallel. The configuration of FIG. 2 has the advantage over that of FIG. 1 that power failures other than a complete input line failure can be detected and backup power can be supplied to the load under such conditions, but the time required to de-energize the coil 25 and switch the mechanical switch 28 from its first position to its second position still results in a substantial interruption of power to the load, typically at least 4 or 5 milliseconds and often 10 to 50 milliseconds or more, which cannot be tolerated by many types of critical loads as discussed above.
In high performance backup power systems, which may be denoted as true uninterruptible power supplies, switching of the power supplied to the load from the main AC power system to an inverter is typically accomplished using high speed static switches. Disruptions of the voltage waveform supplied to the load are virtually nonexistent or of such small magnitude as to be unnoticed by the load. Such systems may also include mechanical relays in the main power path from the AC input to the AC output in addition to the static switches to provide galvanic isolation of the AC input terminals of the system from the inverter when it is operating. Such isolation may be required by electrical codes in many countries. In such systems, the static switches are used to provide quick disconnection of power from the main AC power system and connection of the inverter to supply power to the load with a subsequent opening of the relay contacts to provide the galvanic isolation. An example of such a backup uninterruptible power system is shown in U.S. Pat. No. 5,315,533 to Stich, et al. The static switch in the main power path is used to very quickly disconnect the AC input power from the output as the inverter is being turned on. Because of the switching speed of the static switches, the potential interruptions of the AC voltage waveform applied to the load will be a small fraction of a 60 Hz cycle.